Amplifier circuit designed for use in a bipolar integrated circuit, for amplifying an input signal selected by a switch circuit

ABSTRACT

An amplifier circuit having a switch (SW), wherein the switch selects one of a plurality of input voltage-to-current converting amplifiers (GM1 to GMn), and the input amplifier, thus selected, converts an input signal into a current. This current is added to a feedback signal current. Hence, even if the signals input to the voltage-to-current converting amplifiers have different DC potentials, the DC potential difference does not appear in the output. The DC potential of the output signal of the amplifier circuit is determined by the reference voltage (V REF ) of a feedback voltage-to-current amplifier (GM NF ) and hence remains constant at all times.

TECHNICAL FIELD

The present invention relates to an amplifier circuit which has a switchand a plurality of amplifiers, each designed to amplify an input signalwhen selected by operating the switch, and more particular to anamplifier circuit having a switch, which is designed for use in abipolar integrated circuit.

BACKGROUND ART

A conventional amplifier circuit having a switch has the structure shownin FIG. 1. QA0 and QB0 are mute-mode NPN transistors which have theiremitters connected together, thus forming a mute-mode differential pair.QA1 and QB1, QA2 and QB2, . . . QAn and QBn are input NPN transistors,every two of which have their emitters coupled together, thus forming aninput differential pair. The input transistors QA1, QA2, . . . QAn havetheir collectors connected together, forming a node (the node beingdenoted by A). The input transistors QB1, QB2, . . . QBn have theircollectors connected together, forming a node (the node being denoted byB).

The bases of the input transistors QA to QAn are connected to areference voltage terminal V_(REF) by input-biasing resistors R1 to Rn,respectively, and are also connected to input terminals IN1 to INn,respectively. The bases of the input transistors QB1 to QBn areconnected to a negative feedback terminal NF.

The base of the mute-mode transistor QA0 is connected to the referencevoltage terminal V_(REF). The base of the transistor QB0 is coupled tothe negative feedback terminal NF.

The circuit has a switch SW which can be connected to, and thus selects,the emitter node of the mute-mode transistors QA0 and QB0, that of theinput transistors QA1 and QB1, that of the input transistors QA2 andQB2, that of the input transistors QAn and QBn, or that of the two inputtransistors forming any other intervening pair. A constant-currentsource I1 is connected between the selected emitter node and the groundpotential terminal GND.

The collectors of NPN transistors Q1 and Q2, which form a currentmirror, are connected to the nodes A and B, respectively. A resistor RE1is connected between the emitter of the transistor Q1 and the a Vccpower-supply terminal. A resistor RE2 is connected between the emitterof the transistor Q2 and the Vcc power-supply terminal. The node B isconnected to an output terminal OUT by an emitter follower circuitcomposed of an NPN transistor Q3 and a constant-current source I2connected to the emitter of the transistor Q3. A feedback circuitcomprised of resistors RNF1 and RNF2 and a capacitor CNF is connectedbetween the output terminal OUT and the negative feedback terminal NF.

Input terminals IN1 to INn are coupled to input-signal sources S1 to Snby coupling capacitors C1 to Cn, respectively.

In the amplifier circuit described above, which has the switch SW, theinput-transistors pairs (QA1, QB1), (QA2, QB2), . . . (QAn, QBn) amplifythe input signals input from the input-signal sources S1 to Sn to theinput terminals IN1 to INn. The output of the input-transistor pair,which has been selected by the switch SW, is supplied to the outputterminal OUT via the emitter follower circuit. The gain GV of theamplifier circuit having the switch SW is determined by the constant ofthe feedback circuit, as is represented by the following equation:

    GV=(RNF1+RNF2)/RNF1                                        (1)

When the switch SW selects the mute-mode transistor pair (QA0, QB0), thegain GV is "0," and no outputs will be supplied from the amplifiercircuit.

As has been described, the amplifier circuit has coupling capacitors C1to Cn and an input-biasing resistors R1 to Rn. These elements are usedfor the purpose of preventing the DC potential of the output signal fromchanging when the switch SW is changed over. Were the input signalsinput directly to the bases of the input transistors QA1 to QAn, the DCpotential of the output signal would vary when the switch SW is changedover if the DC potentials V1 to Vn of the input signals are different.The use of the coupling capacitors C1 to Cn and the input-biasingresistors R1 to Rn, however, means an increase in the number of elementsforming the amplifier circuit.

Further, to form a mute-mode pair, the mute-mode transistors QA0 and QB0are indispensable.

The gain GV of the amplifier circuit having the switch SW is set at afixed value. In other words, it cannot be set at any value suitable foreach input signal.

The conventional amplifier circuit having a switch has three problems.First, it comprises many elements. Second, it needs two mute-modetransistors for constituting a mute-mode pair. Third, its gain cannot beindividually set for each input signal.

The present invention has been made in order to solve the problemsdescribed above. Its object is to provide an amplifier circuit having aswitch, which comprises a relatively small number of elements, can havea gain individually set for each input signal, and can operate in mutemode without requiring mute-mode transistors.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided an amplifiercircuit which comprises a plurality of input voltage-to-currentconverting amplifiers, an adder circuit, a current-to-voltage convertingamplifier, a feedback voltage-to-current converting amplifier, and aswitch. Each of the input voltage-to-current amplifiers has two inputterminals, one of which is coupled to an input-signal source having a DCpotential source, and the other of which is connected to the DCpotential source. These amplifiers have output terminals connectedtogether, and are applied with different DC voltages from the DCpotential sources. The current output flowing at the node of the outputterminals of these amplifiers is supplied to one input terminal of theadder circuit. The current output by the adder circuit is supplied tothe current-to-voltage converting amplifier. The output by thecurrent-to-voltage converting amplifier is applied via a feedbackcircuit to one input terminal of the feedback voltage-to-currentconverting amplifier circuit. A reference potential is applied to theother input terminal of the feedback voltage-to-current convertingamplifier circuit. The current output by the feedback voltage-to-currentconverting amplifier is supplied to the other input terminal of theadder circuit. The switch is operated to select and connects one of theinput voltage-to-current amplifiers to an operation current source.

In the amplifier circuit described above, the one of the inputvoltage-to-current amplifiers is selected by operating the switch, theselected voltage-to-current converting amplifier converts an inputsignal to a current, and the current, thus obtained, is added to afeedback-signal current. Hence, although the signals input to thevoltage-to-current converting amplifiers have different DC potentials,the DC potential difference does not appear in the output when any ofthese amplifier is switched to another by means of the switch. As aresult, the DC potential of the output signal of any amplifier selectedis determined by only the reference potential applied to the feedbackvoltage-to-current converting amplifier circuit, and therefore remainsunchanged at all times.

The input voltage-to-current converting amplifiers can have gainsrespectively set for input signals, merely by imparting differenttransfer conductances gm to the input amplifiers.

When the switch selects none of the input voltage-to-current convertingamplifiers, the gains for the signals input to these amplifiers arezero, and the amplifier circuit is set in the mute mode. While thecircuit remains in the mute mode, the DC output potential of the circuitis equal to the reference voltage applied to the feedbackvoltage-to-current converting amplifier, whereby there are no changes inthe output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a conventional amplifier circuithaving a switch.

FIG. 2 is a block diagram representing the basic structure of anamplifier circuit having a switch, according to the present invention.

FIG. 3 is a circuit diagram illustrating an embodiment of the amplifiercircuit shown in FIG. 2.

FIG. 4 is a circuit diagram showing another embodiment of the amplifiercircuit shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described in detail,with reference to the accompanying drawings.

FIG. 2 shows the basic structure of an amplifier circuit having aswitch, according to the present invention. INA1 to INAn designate inputterminals connected to input-signal sources S1 to Sn which have DCpotentials V1 to Vn, respectively. GM1 to GMn denote inputvoltage-to-current converting amplifiers for receiving, at one inputterminal, input signals from the input terminals INA1 to INAn andamplifying these input signals. The amplifiers GM1 to GMn receive, atthe other input terminal, reference voltages V1 to Vn which are equal tothe DC potentials V1 to Vn of the input-signal sources S1 to Sn. One ofthe input amplifiers GM1 to GMn is selected by a switch SW and isthereby connected to an operation-current source I1. The current outputby the selected input amplifier is supplied to one input terminal of anadder circuit 10. A current-to-voltage converter circuit 11 having acurrent-to-voltage conversion gain K converts the output current of theadder circuit 10 into a voltage, which is supplied to an output terminalOUT. A feed back circuit 12 having a feedback ratio β is connectedbetween the output terminal OUT and a negative feedback terminal NF. Theoutput voltage of the converter circuit 11 is applied to the invertinginput terminal (-) of a feedback amplifier GM_(NF) through the feedbackcircuit 12 and the negative feedback terminal NF. A reference voltageV_(REF) is applied to the non-inverting input terminal (+) of theamplifier GM_(NF). The current output by the amplifier GM_(NF) issupplied to the other input terminal of the adder circuit 10.

In the amplifier circuit having a switch, the switch SW selects one ofthe input amplifier circuits GM1 to GMn, the input signal of the inputamplifier selected is converted into a current, and the current thusobtained is added to the current output by the feedback amplifierGM_(NF). Hence, although the signals input to the input amplifiers GM1to GMn have different DC potentials V1 to Vn, the difference among theDC potentials V1 to Vn does not influence the output currents of theseamplifiers when any of these amplifier is switched to another by theswitch SW. As a result, the DC potential of the output signal of anyamplifier selected is determined by only the reference potential V_(REF)applied to the feedback amplifier GM_(NF), and therefore remainsunchanged at all times.

Hence, input signals can be directly input to the input amplifiers GM1to GMn. Unlike the conventional amplifier circuit shown in FIG. 1, theamplifier circuit needs neither elements equivalent to the couplingcapacitors C1 to Cn nor element equivalent to the input-biasingresistors R1 to Rn. Thus, the amplifier circuit of the inventioncomprises a very small number of elements.

In addition, the input amplifiers GM1 to GMn can have different gainsfor input signals, respectively, merely by imparting different transferconductances gm to the input amplifiers.

Further, when the switch SW selects none of the input amplifiers GM1 toGMn, the gains for the signals input to these amplifiers are zero, andthe amplifier circuit is set in the mute mode. While the circuit remainsin the mute mode, the DC output potential of the circuit is equal to thereference voltage V_(REF) applied to the feedback amplifier GM_(NF),whereby there are no changes in the output voltage.

Hence, the amplifier circuit need not have elements corresponding to themute-mode transistors (QA0, QB0) which are indispensable to theconventional amplifier circuit shown in FIG. 1.

FIG. 3 shows an embodiment of the amplifier having a switch, which isillustrated in FIG. 2. QA1 and AB1, QA2 and QB2, . . . QAn and QBn areinput NPN transistors, every two of which have their emitters coupledtogether, thus forming an input differential pair. The pairs of NPNtransistors, thus formed, are the input amplifiers GM1 to GMn. The inputtransistors QA1, QA2, . . . QAn have their collectors connectedtogether, forming a node (the node being denoted by A). The inputtransistors QB1, QB2, . . . QBn have their collectors connectedtogether, forming a node (the node being denoted by B). The emitters ofthe input transistors forming each pair, (i.e., QA1, QB1; QA2, QB2; . .. QAn, QBn) are connected to each other, thus forming a node. The switchSW is operated, selecting the emitter node of one of theinput-transistor pairs. The variable constant-current source I1 isconnected between the selected emitter node and the ground potentialterminal GND. The bases of transistors QA1 to QAn are coupled to theinput terminals INA1 to INAn, respectively. The reference voltages V1 toVn are applied to the bases of the transistors QB1 to QBn, respectively.The input terminals INA1 to INAn are connected to the input-signalsources S1 to Sn having the DC potentials V1 to Vn, respectively.

A resistor R4 and the emitter-collector path of an PNP transistor Q4 areconnected between a Vcc power-supply terminal and the node B. The baseand collector of the PNP transistor Q4 are connected together. The baseof a PNP transistor Q3 is coupled to the base of the PNP transistor Q4.A resistor R3 is connected between the emitter of the transistor Q3 andthe Vcc power-supply terminal. These resistors R4 and R3 and thetransistors Q4 and Q3 constitute a first current mirror circuit.Similarly, a resistor R5 and the emitter collector path of a PNPtransistor Q5 are connected between the Vcc power-supply terminal andthe node A. The base and collector of the PNP transistor Q5 areconnected to each other. The base of a PNP transistor Q6 is coupled tothe base of the transistor Q5. A resistor R6 is coupled between theemitter of the transistor Q6 and and Vcc power-supply terminal. Theresistors R5 and R6 and the transistors Q5 and Q6 constitute a secondcurrent mirror circuit.

The collector-emitter path of an NPN transistor Q8 and a resistor R7 areconnected between the collector of the transistor Q3 and the groundpotential terminal GND. The collector and base of this transistor Q8 areconnected to the base and emitter of an NPN transistor Q7, respectively.The collector of the transistor Q7 is coupled to the Vcc power-supplyterminal. The collector-emitter path of an NPN transistor Q9 and aresistor R8 are connected between the collector of the transistor Q6 andthe ground potential terminal GND. The base of the transistor Q9 iscoupled to the base of the transistor Q8. The transistors Q7 to Q9 andthe resistors R7 and R8 constitute a third current mirror circuit.

The collector output of the transistor Q6 is connected to the outputterminal OUT by the emitter follower circuit which comprises an NPNtransistor Q10 and a third constant-current source I3 connected betweenthe emitter of the NPN transistor Q10 and the ground potential terminalGND.

A feedback circuit 12 comprising resistors R2 and R1 and a capacitor C1is connected between the output terminal OUT and a negative feedbackterminal NF.

Q1 and Q2 designate NPN transistors, the emitters of which are connectedtogether, and which therefore form a differential pair. Thesetransistors have their collectors connected to the collectors of thetransistors Q4 and Q5, respectively, and their emitters connected toeach other. A second constant-current source I2 is coupled between theemitter node of the transistors Q1 and Q2 and the ground potentialterminal GND. The base of the transistor Q1 is coupled to the negativefeedback terminal NF (i.e., the node of the resistors R2 and R1 of thefeedback circuit 12). A reference voltage V_(REF) is applied to the baseof the transistor Q2.

The transistors Q1 and Q2 and the second constant current source I2constitute a feedback amplifier GM_(NF). The resistors R3 to R8 and thetransistor Q3 to Q9 constitute the adder circuit 10. The emitterfollower circuit, which comprises the transistor Q10 and the thirdconstant-current source I3, is the current-to-voltage converting circuit11.

In the amplifier circuit described above, the switch SW selects theemitter node of one of the input transistor pairs (QA1, QB1), (QA2,QB2), . . . (QAn, QBn), and connects the selected emitter node to thevariable constant-current source I1. As a result of this, the transistorpair, the emitter node of which has been selected, i.e., the inputamplifier formed of this pair, is selected.

The transfer conductance gmi (i=1 to n) of the selected amplifier is:

    gmi=I1/(2·VT)                                     (2)

where VT is thermal voltage.

The transfer conductance gm_(NF) of the feedback amplifier GM_(NF) is:

    gm.sub.NF =I2/(2·VT)                              (3)

The current-to-voltage conversion gain K and feedback ratio β of theamplifier circuit is as follows, assuming that the NPN transistor Q10has a current amplification factor h_(FE) :

    K=h.sub.FE (R1+R2)                                         (4)

    β=R1/(R1+R2)                                          (5)

Hence, the amplifier circuit having a switch, which is illustrated inFIG. 3, has a gain GV given by: ##EQU1##

The input amplifier selected and connected to the variableconstant-current source I has, for the input signal, gain GV defined byequation (6). By contrast, the any other input amplifier, which is notselected and not connected to the variable constant-current source I1,has a transfer conductance gm of "0" and a gain GV of "0." Hence, onlythe signal input to the selected input amplifier is amplified and outputfrom the amplifier circuit; the signal input to any not selected inputamplifier is not output at all. The circuit of FIG. 3 thereforefunctions an amplifier circuit with a switch.

The input amplifiers GM1 to GMn can have different gains GV for inputsignals, respectively, merely by varying the currents from the variableconstant-current source I1 to the input amplifiers GM1 to GMn.

The DC potential at the output of the amplifier circuit is equal to thereference voltage V_(REF) which is applied to the feedback amplifiercircuit GM_(NF).

Further, when the switch SW selects none of the input amplifiers GM1 toGMn, the gain GV is "0," and the input signal does not appear in theoutput. Hence, the amplifier circuit is set in the mute mode. While thecircuit remains in the mute mode, the DC output potential of the circuitis equal to the reference voltage V_(REF) applied to the feedbackamplifier GM_(NF).

FIG. 4 illustrates another embodiment of this invention, which isidentical to the circuit shown in FIG. 3, except for the followingrespects. First, the emitter of the input transistors of each pair,i.e., (QA1, QB1), (QA2, QB2), . . . or (QAn, QBn), have their emittersconnected together by two resistors REi (i=1to n). Second, the emittersof NPN transistors Q1 and Q2 are connected to each other by tworesistors RE_(NF). Therefore, in FIG. 4, the same symbols designate thecomponents identical to those shown in FIG. 3.

In the amplifier circuit with a switch, shown in FIG. 4, the transferconductance gmi (i=1 to n) of the amplifier selected by operating theswitch SW is:

    gmi=1/{(2·VT/I1)+REi}                             (7)

The transfer conductance gm_(NF) of the feedback amplifier GM_(NF) is:

    gm.sub.NF =1/{(2·VT/I1)+RE.sub.NF}                (8)

Hence, the gain GV of the amplifier circuit having a switch, which isillustrated in FIG. 4, is: ##EQU2##

The input amplifiers GM1 to GMn can have different gains GV for inputsignals, respectively, merely by imparting different resistances REi tothe input amplifiers.

The amplifier circuit of FIG. 4 has a broad dynamic range, and canreduce the restrictions on the input signal. This is because an emitterresistor REi is incorporated in each of the input amplifier circuits GM1to GMn.

As has been described, the present invention can provide an amplifiercircuit having a switch, which comprises a small number of elements,which can have a gain individually set for each input signal, and whichcan operate in mute mode without requiring mute-mode transistors.

INDUSTRIAL APPLICABILITY

Because of the advantages described above, the amplifier circuit havinga switch, according to the invention, is useful in the case where it isrequired that the circuit have a high integration density, have a gainindividually set for each of the input signals, or have no mute-modetransistors.

I claim:
 1. An amplifier circuit having a plurality of inputvoltage-to-current converting amplifiers, each having two inputterminals, one of which is coupled to an input-signal source having a DCpotential source, and the other of which is connected to the DCpotential source, said input amplifiers having output terminalsconnected together, and having applied thereto different DC voltagesfrom the DC potential source, said amplifier circuit comprising:an addercircuit having two input terminals, one of which is connected to receivethe current output flowing from a node of the output terminals of theinput voltage-to-current converting amplifiers; an additionalcurrent-to-voltage converting amplifier connected to receive the currentoutput by said adder circuit, for converting the current into a voltage;a feedback voltage-to-current converting amplifier circuit having twoinput terminals, one of which is connected to receive an output appliedfrom said additional current-to-voltage converting amplifeir through afeedback circuit, and the other of which is connected to receive areference potential, for generating and supplying a current to the otherinput terminal of said adder circuit; and a switch circuit for selectingand connecting one of said input voltage-to-current amplifiers to anoperation current source.
 2. The amplifier circuit according to claim 1,wherein said operation current source is a variable current source. 3.The amplifier circuit according to claim 1 or claim 2, wherein saidfeedback voltage-to-current converting amplifier comprises bipolartransistors forming a differential pair having collectors connected tosaid adder circuit, and emitters connected together by resistors,forming an emitter node connected to a constant current source.